From Halopedia, the Halo wiki
"Carney Hole" is the name given to special spots in Halo 2, Halo 3, and Halo: Reach multiplayer levels that would give the player a huge tactical advantage over other players. Each was designed by Bungie employee, Chris Carney.
Carney Holes are included in many of the multiplayer levels. Some are hard to find, while others are wide out in the open. Some will contain weapons, a good view of the battlefield (and thus a good place to snipe), or just a very good place to defend.
List of Carney Holes